Structural parts having high permanent strength



Patented June 8, 1937 rSA-TENT creme I 2,083,208 STRUCTURAL marsnsvmeflqnlon PER;

MANENT STRENGTH Walter Tofaute, Essen, Germany, assignor to TheNitralloy Corporation, a corporation of Delaware N0 Drawing. ApplicationSeptember 17, 1934,.

Serial No. 744,365. in Germany September 18,

3 Claims.

This invention relates to structural parts, such as screws, bolts andtubes adapted for use in power plant installations and similarsituations where high resistance to pulling or tension stresses atelevated temperatures is a desideratum.

It has been found that the strength of iron and steel alloys at elevatedtemperatures and their resistance to creep at such temperatures can begreatly increased by subjecting them to an appropriate nitridingtreatment. This resistance to stress and to creep at elevatedtemperatures is referred to hereinafter as permanent strength and is tobe understood as the load in kg./m1n.

which an iron or steel alloy will support at a temperature of 500 C.without showing an extensibility or rate of creep of more than 0.0005%per hour in the 25 to 35 hour period of the tension test.

In the conduct of the nitriding treatment for the purpose of impartinghigh permanent strength, it is not important that a high surfacehardness be produced. The purposes of the present invention areaccomplished by so conducting the nitriding operation, or so choosingthe iron or steel alloy that is to be treated, that the depth of thenitride case is relatively high as compared with the present nitridingpractice. It has been found that many iron and steel alloys heretoforeregarded as unsuitable for nitriding because the obtainable surfacehardness is relatively low, may be caused to acquire a high permanentstrength when nitrided. In other words, although the surface hardnessimparted by nitriding is relatively low the nitride depth that can beimparted is such as to insure a satisfactory permanent strength andresistance to creep at high temperatures. For example, manyaluminum-free chromium steels show a relatively low hardness whennitrided, but nevertheless acquire a nitride case of relatively greatdepth and consequent high permanent strength. As an example, a steelalloy of the following analysis has been tested:

A test piece of this steel with a diameter of 18 mm. was cooled from 900C. in air, drawn to 700 C. and then nitrided at 500 C. for hours bytreatment with ammonia gas. This test piece showed a nitride depth of1.1 mm. and a permanent strength of 21.8 kg./mm. A test piece of theabove steel alloy heat treated in like manner but not nitrided showed apermanent strength of 13.8 kg./mm'.

Steel alloys which when nitrided at a normal nitriding temperature, sayat about 500 C., form a very hard but comparatively thin nitride layer,as for example, the chromium-aluminum steel alloys, undergo whennitrided at such a temperature little or no substantial improvement ofthe permanent strength. It has been found that when these alloys arenitrided under conditions such as to produce a relatively great nitridedepth and at the same time decreased hardness (for example, of about600-800 according to the Firth hardometer), a highly satisfactoryincrease in permanent strength is obtained. This result may be obtainedby carrying on the nitriding step at a relatively high temperature, forexample, at

- about 550 to- 600 C., or, if desired, at an even higher temperature upto about 800 C. In order to illustrate this, there are recited below theresults of tests on steel alloys of the following compositions:

C Si Mn Or Al Mo Steel I 0. 33 0. 31 0. 76 l. 00 1. 08 0. 25 Steel II 0.29 0. 16 0. 38 l. 39 1. 18

C. in oil and drawing to 650 C. showed a permanent strength of 4 kg./mm.Another test piece after a similar heat treatment followed 'by nitridingat 560 C. for 100 hours showed a permanent strength of 10.4 kg./mm. witha nitride depth of 0.97 mm.

For producing structural parts according to the present invention, ironand steel alloys that are especially adapted for the purpose are thosewhich have a carbon content up to 1.5% and contain up to about 3%silicon, up to 3% manganese, up to 20% chromium, up to 3% aluminum, upto 3% molybdenum, tungsten, titanium, vanadium or zirconium, and. up to20% nickel or cobalt. The hardness of the nitrided alloys in thefollowing claims is expressed in terms of measurements made with a Firthhardorneter.

I claim:

1. A structural part subject in use to prolonged tension stresses atelevated temperatures, formed of a ferrous-base alloy having a nitridedcase having a hardness between 600 and 800 penetrating to a depth ofabout 1 mm. or more, and characterized by a permanent strength in excessof an identical unnitrided part formed of the same alloy.

2. A structural part subject to prolonged tension stresses and adaptedfor use at elevated temperatures, formed of an aluminum-freechromium-containing steel alloy having a nitrided case having a hardnessbetween 600:-a nd 800,

penetrating to a depth of about 1 mm. or more, and characterized by apermanent strength in excess of an identical unnitrided part formed ofthe same alloy.

3. A structural part subject to prolonged tension stresses and adaptedfor use at elevated temperatures, formed of a ferrous-base alloycontaining chromium and aluminum in effective proportions but notexceeding about 20% of chromium and about 3% of aluminum, said parthaving a nitrided case having a hardness between 600 and 800 penetratingto a depth of about 1 mm. or more, and characterized by a permanentstrength in excess of an identical unnitrided part formed of the samealloy.

WALTER TOFAUTE.

